The invention relates to semiconductor integrated circuits, and more particularly to mixed signal integrated circuits and fabrication methods.
Mixed signal integrated circuits (which contain both analog and digital devices) typically include CMOS and bipolar transistors plus capacitors and resistors. Such integrated circuits compromise performance of the various devices in order to reduce process complexity. However, circuits such as 12-bit analog-to-digital converters require precision capacitors with minimal capacitance dependence upon applied voltage. But known mixed signal processes do not provide precision capacitors without undue complexity.
The invention provides a precision capacitor in a mixed signal process by use of a single polysilicon layer for both MOS gates and capacitor plates but with diffusive doping in the capacitor plates portion and implant doping in the polysilicon gates portion.
This has the advantages of high doping to limit variation of capacitance with applied voltage to the capacitors and avoidance of diffusive doping affecting MOS channel regions.